Packing yarns are generally used as seals for shafts and other mechanical devices. Packing yarns during use in their environments undergo a variety of stresses and strains which cause fiber breakage. Fibers with improved elongability are capable of better withstanding these stresses and strains.
When carbon and graphitic fibers are produced from a stabilized acrylic precursor fiber, the extensibility or percent extension is typically in the range of 1.25 to 1.9%, depending upon the heat treatment, the degree of carbonization or graphitization and modulus of the fiber. Typical linear graphitic or carbon fibers are produced by processing tows of from 1,000 to 320,000 filaments through a zone temperature furnace which heat treats the fiber in a temperature range of from about 300.degree. C. graduated rapidly up to about a 1050.degree. to 1100.degree. C. temperature range. This treatment is generally followed by a subsequent heat treatment in a high temperature furnace where the fiber is taken up at a temperature of from about 1400.degree. to 2400.degree. C. The heat treatment is carried out under tension even in a low temperature furnace. That is, the fibers are suspended through the furnace with sufficient tension to pull the fiber tows through the furnace and keep them off the floor or bottom of the furnace. It is especially advantageous if one desires to utilize the carbonaceous fibers as packing yarn to have a percent elongation of from about 2.5% or greater. When a partially carbonized fiber, that is, a fiber which still has a nitrogen content of from 12 to 20%, is heat treated at from 550.degree. to 650.degree. C. under tension, the extensibility of the fiber is only 2.0% or less. This low extensibility is insufficient for use as packing yarn without encountering considerable fiber breakage when placed into a packing gland or recess.
Ideal packing yarns are those having the favorable characteristics of 1) not being friable, 2) resistant to chemicals, 3) high strength, 4) good lubricity, 5) good compressive properties, and 6) can be used at temperatures over 350.degree. C.
Current fibers which are not used for packing yarn, such as Kevlar, Nomex and Teflon, cannot be used at temperatures over 350.degree. C. because they undergo decompostion or soften. Even with glass fibers it is advisable not to exceed temperatures over 400.degree. C. Asbestos is suitable for use as packing yarn but it is being replaced in most systems.
U.S. Pat. No. 4,347,297 to Mishima et al discloses a process for the preparation of carbon fibers by two preoxidation treatments of polyacrylonitrile fibers under tension and the carbonizing of the oxidized fibers under tension.
U.S. Pat. No. 4,279,612 to Saji et al discloses a method for producing carbon fibers which includes the step of thermally stabilizing the fibers under tension before heat treatment to carbonize the fibers.
U.S. Pat. No. 3,541,582 to Fainborough et al which discloses the preparation of woven carbon cloth by first oxidizing continuous yarns of polymeric fibers while under tension. The carbonization step is performed either while under tension or without tension. However, the woven cloth inherently places the fibers under tension.
U.S. Pat. No. 4,837,076 to Mc Cullough et al which is herein incorporated by reference discloses a process for preparing non-linear carbonaceous fibers, yarns and tows having a reversible deflection greater than 1.2:1. The conditions for heat treatment described in the patent can be used to provide similar electrical conductivity to linear fibers. However, non-linear fibers form yarns that are too bulky for use as packing yarn.
The term "carbonaceous fibers" is understood to mean fibers, which have been heated to have an increased carbon content, namely, a carbon content of greater than 65% as a result of an irreversible chemical reaction.
It is to be understood that the percentage stated relate to percent by weight of the total composition unless stated otherwise.